1. Field of the Invention
This invention relates to mobile telephones which operate through a radio network and in particular it relates to a security function for restricting the use of individual mobile telephones to radio networks of selected operators.
2. Description of the Related Art
The mobile telephone for which the invention is provided is a digital radio transceiver programmed to operate under the control of a central processing unit (CPU).
Owing to the variations in pricing policy of network operators, the nominal selling price of mobile telephones is often heavily discounted in favor of a higher call tariff. The possibility exists therefore of a user obtaining at very low or even no cost, a mobile telephone for use on a specified network with a relatively high call tariff. If the mobile telephone were then to be used on a low call tariff network for which the charge for mobile telephones remained at full selling price then both network operators would be deprived of revenue. It is important to network operators therefore that use of mobile telephones be restricted to specified networks.
On GSM (Global System for Mobile communication) networks the restriction of mobile telephones to specified networks is sometimes called `network lock` and this definition of network lock shall be used herein. Network lock for GSM is implemented by ensuring that the mobile telephone will only operate if a SIM (Subscriber Identity Module) belonging to the appropriate network is inserted. The SIM is a smartcard which is attached to the mobile telephone and contains information about the user's subscription (see TS GSM 11.11, `Digital Cellular Telecommunications System; Specification of Subscriber Identity Module-Mobile Equipment Interface, published by European Telecommunications Standards Institute).
There is a need for a convenient means to be provided to enable the network operator or the agents of the operator to unlock a mobile telephone, for instance at the end of a service contract. It is preferable that this can be carried out without the participation of the manufacturer. The present method is not conductive to this, since it relies on the administration of secret numbers indexed in accordance with mobile telephone equipment numbers.